Problem: Jessica did 7 fewer sit-ups than Tiffany in the evening. Tiffany did 35 sit-ups. How many sit-ups did Jessica do?
Tiffany did 35 sit-ups, and Jessica did 7 fewer than that, so find the difference. The difference is $35 - 7$ sit-ups. She did $35 - 7 = 28$ sit-ups.